Benzoin condensation MCQs With Answer

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Benzoin condensation MCQs With Answer offers B. Pharm students a focused, exam-ready review of the benzoin condensation reaction, mechanism, catalysts, and pharmaceutical relevance. This concise introduction covers cyanide and thiamine catalysis, the Breslow intermediate, cross‑benzoin selectivity, asymmetric variants, laboratory conditions, safety considerations, and analytical characterization. Well-suited for organic chemistry and medicinal chemistry courses, these questions reinforce mechanistic understanding, synthetic utility, and practical troubleshooting. Keywords included: benzoin condensation, Benzoin condensation MCQs With Answer, cyanide catalysis, thiamine, NHC catalysis, acyl anion equivalent, and B. Pharm students. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the main product of the benzoin condensation of benzaldehyde?

  • Benzoin
  • Benzil
  • Benzoic acid
  • Benzyl alcohol

Correct Answer: Benzoin

Q2. Which catalyst is classically used in laboratory benzoin condensation?

  • Sodium borohydride
  • Potassium permanganate
  • Cyanide ion
  • Palladium on carbon

Correct Answer: Cyanide ion

Q3. Which biological cofactor catalyzes benzoin-like reactions in enzymatic systems?

  • NADH
  • Vitamin B12
  • Thiamine (vitamin B1)
  • Biotin

Correct Answer: Thiamine (vitamin B1)

Q4. What is the key reactive intermediate in thiamine-catalyzed benzoin reactions?

  • Enamine intermediate
  • Breslow intermediate
  • Keto-enol tautomer
  • Carbocation intermediate

Correct Answer: Breslow intermediate

Q5. Which statement best describes the acyl anion equivalent in benzoin condensation?

  • A stable carboxylate anion
  • An anionic species behaving like an acyl anion
  • A neutral radical centered on oxygen
  • A cationic acyl transfer species

Correct Answer: An anionic species behaving like an acyl anion

Q6. Which reagent can oxidize benzoin to benzil?

  • Nitric acid
  • Sodium borohydride
  • Hydrochloric acid
  • Tetramethylsilane

Correct Answer: Nitric acid

Q7. In cyanide-catalyzed benzoin condensation, what is the first step?

  • Elimination of cyanide
  • Nucleophilic attack of cyanide on the carbonyl carbon to form a cyanohydrin
  • Oxidation of benzaldehyde
  • Protonation of carbonyl oxygen

Correct Answer: Nucleophilic attack of cyanide on the carbonyl carbon to form a cyanohydrin

Q8. Cross-benzoin condensation refers to:

  • Condensation between two identical aldehydes only
  • Condensation of an aldehyde with a ketone
  • Condensation between two different aldehydes to give a mixed benzoin
  • Polymerization of aldehydes

Correct Answer: Condensation between two different aldehydes to give a mixed benzoin

Q9. Which factor increases the likelihood of cross‑benzoin selectivity?

  • Using two strongly electron-rich aldehydes
  • Using one activated (electron-poor) aldehyde and one unactivated aldehyde
  • Increasing water content dramatically
  • Performing the reaction at very high temperature

Correct Answer: Using one activated (electron-poor) aldehyde and one unactivated aldehyde

Q10. Which modern organocatalyst class can replace cyanide in benzoin-type reactions?

  • Alkali metals
  • N‑heterocyclic carbenes (NHCs)
  • Transition metal hydrides
  • Lewis acidic boranes

Correct Answer: N‑heterocyclic carbenes (NHCs)

Q11. What safety concern is most associated with classical benzoin condensation using cyanide?

  • Flammability of cyanide salts
  • Toxicity and potential release of hydrogen cyanide
  • Explosive formation of benzoin
  • Strong radioactivity

Correct Answer: Toxicity and potential release of hydrogen cyanide

Q12. Which solvent is commonly used for cyanide-catalyzed benzoin condensation?

  • Water only
  • Nonpolar hexane
  • Polar protic solvents like ethanol
  • Concentrated sulfuric acid

Correct Answer: Polar protic solvents like ethanol

Q13. Which functional groups on benzaldehyde decrease reactivity in benzoin condensation?

  • Electron-withdrawing groups like nitro
  • Electron-donating groups like methoxy
  • Free hydroxyl groups that can hydrogen-bond strongly
  • Halogens with weak inductive effects

Correct Answer: Electron-donating groups like methoxy

Q14. Which spectroscopic feature confirms formation of benzoin?

  • Disappearance of aldehyde C–H stretch at ~1700 cm⁻¹
  • Appearance of both hydroxyl (O–H) stretch and a carbonyl (C=O) stretch in IR
  • Loss of aromatic signals in NMR
  • Formation of a nitrile peak at ~2250 cm⁻¹

Correct Answer: Appearance of both hydroxyl (O–H) stretch and a carbonyl (C=O) stretch in IR

Q15. What is the regiochemical outcome when two different aldehydes are used without selectivity control?

  • Only one cross-product forms exclusively
  • Equal mixture of two cross-products and two homocoupled products may form
  • No reaction occurs
  • Only homocoupled products form

Correct Answer: Equal mixture of two cross-products and two homocoupled products may form

Q16. Which step regenerates the cyanide catalyst in the classical mechanism?

  • Protonation of the Breslow intermediate
  • Elimination of cyanide after formation of benzoin
  • Oxidation by air
  • Condensation with another cyanide

Correct Answer: Elimination of cyanide after formation of benzoin

Q17. Which property of the Breslow intermediate is crucial for benzoin formation?

  • Its electrophilicity at carbonyl carbon
  • Its ability to act as an acyl anion equivalent (nucleophilic carbon center)
  • Being a stable free radical
  • Forming a strong ion pair with solvent

Correct Answer: Its ability to act as an acyl anion equivalent (nucleophilic carbon center)

Q18. Which catalyst loading is typical for NHC-catalyzed benzoin reactions?

  • Stoichiometric amounts (1 equivalent)
  • High catalytic loading (>50 mol%)
  • Low catalytic loading (1–20 mol%)
  • No catalyst required

Correct Answer: Low catalytic loading (1–20 mol%)

Q19. Asymmetric benzoin condensation aims to:

  • Produce the racemic benzoin exclusively
  • Generate benzoin with a specific enantiomeric excess
  • Prevent benzoin formation entirely
  • Convert benzoin to benzil selectively

Correct Answer: Generate benzoin with a specific enantiomeric excess

Q20. Which class of chiral catalysts is used for asymmetric benzoin reactions?

  • Chiral NHCs
  • Achiral Lewis acids
  • Simple inorganic bases only
  • Radical initiators

Correct Answer: Chiral NHCs

Q21. Which outcome indicates intramolecular benzoin condensation?

  • Formation of linear dimers only
  • Cyclization to form cyclic α-hydroxy ketones
  • Complete polymerization
  • Alkylation at the aromatic ring

Correct Answer: Cyclization to form cyclic α-hydroxy ketones

Q22. Which reagent pair is commonly used to quench cyanide after a benzoin reaction for safety?

  • Hydrochloric acid and sodium chloride
  • Sodium hypochlorite or hydrogen peroxide to oxidize cyanide
  • Sodium borohydride and methanol
  • Pyridine and acetic anhydride

Correct Answer: Sodium hypochlorite or hydrogen peroxide to oxidize cyanide

Q23. Why are aromatic aldehydes typically used in benzoin condensations?

  • They are more reactive than aliphatic aldehydes due to resonance-stabilized intermediates
  • They cannot form cyanohydrins
  • They are insoluble and precipitate product directly
  • They are nonpolar and avoid side reactions

Correct Answer: They are more reactive than aliphatic aldehydes due to resonance-stabilized intermediates

Q24. Which of the following is NOT a typical side reaction in benzoin condensations?

  • Oxidation of aldehyde to acid
  • Polymerization of aldehyde
  • Reduction to corresponding alcohol by cyanide
  • Formation of benzil by over-oxidation

Correct Answer: Reduction to corresponding alcohol by cyanide

Q25. How does an electron-withdrawing substituent on benzaldehyde affect benzoin yield?

  • Usually decreases yield due to deactivation
  • Has no effect at all
  • Often increases yield by stabilizing the intermediate and favoring cross-coupling
  • Makes the aldehyde insoluble and stops the reaction

Correct Answer: Often increases yield by stabilizing the intermediate and favoring cross-coupling

Q26. Which analytical technique best distinguishes benzoin from benzaldehyde in crude reaction mixture?

  • Thin layer chromatography (TLC) coupled with IR or NMR confirmation
  • Simple visual inspection
  • Melting point measurement only
  • Mass spectrometry without chromatography

Correct Answer: Thin layer chromatography (TLC) coupled with IR or NMR confirmation

Q27. What is the role of a proton shuttle in the benzoin mechanism?

  • To oxidize the product
  • To transfer protons during formation of the nucleophilic intermediate and subsequent steps
  • To form a stable carbocation
  • To act as a radical initiator

Correct Answer: To transfer protons during formation of the nucleophilic intermediate and subsequent steps

Q28. Which statement about benzoin vs acyloin condensation is correct?

  • Benzoin condensation couples aldehydes; acyloin couples esters under reductive conditions
  • Benzoin uses radical chemistry; acyloin uses acid catalysis only
  • Both reactions always require cyanide
  • Acyloin condensation yields α,β‑unsaturated ketones

Correct Answer: Benzoin condensation couples aldehydes; acyloin couples esters under reductive conditions

Q29. Which observation suggests formation of the Breslow intermediate in NMR studies?

  • Appearance of a new downfield aromatic peak only
  • Observation of a proton signal consistent with an enaminol-like species
  • Complete disappearance of all aldehyde signals instantly
  • Formation of a sharp nitrile signal

Correct Answer: Observation of a proton signal consistent with an enaminol-like species

Q30. Which catalyst provides a metal-free route to acyl anion chemistry useful in benzoin-type reactions?

  • Transition metal carbene complexes
  • N‑heterocyclic carbenes (NHCs)
  • Lanthanide triflates
  • Grignard reagents

Correct Answer: N‑heterocyclic carbenes (NHCs)

Q31. Which reagent is commonly used to prepare cyanide catalysts in situ for benzoin reactions?

  • Sodium cyanoborohydride
  • Potassium cyanide or sodium cyanide dissolved in alcohol or water
  • Sodium azide
  • Ammonium chloride

Correct Answer: Potassium cyanide or sodium cyanide dissolved in alcohol or water

Q32. Which condition typically reduces self-condensation and favors cross-benzoin?

  • High concentration of both aldehydes
  • Slow addition of one aldehyde to a solution of the other under catalyst control
  • Strong heating to reflux for long periods
  • Using excess base but no catalyst

Correct Answer: Slow addition of one aldehyde to a solution of the other under catalyst control

Q33. In benzoin condensation, the newly formed C–C bond links which positions of the reactant aldehydes?

  • α-carbon to α-carbon of two carbonyls
  • Carbonyl carbon of one to α-carbon (formerly carbonyl carbon) of the other forming an α‑hydroxy ketone
  • Two oxygen atoms to form an ether
  • Two hydrogen atoms to form H2

Correct Answer: Carbonyl carbon of one to α-carbon (formerly carbonyl carbon) of the other forming an α‑hydroxy ketone

Q34. Which functional group in benzoin can be further oxidized to form benzil?

  • Aromatic ring
  • Hydroxyl group to carbonyl
  • Nitrile group
  • Alkene group

Correct Answer: Hydroxyl group to carbonyl

Q35. Which of the following substrates is least likely to undergo benzoin condensation efficiently?

  • Benzaldehyde
  • p-Nitrobenzaldehyde
  • Formaldehyde
  • p-Anisaldehyde (p-methoxybenzaldehyde)

Correct Answer: p-Anisaldehyde (p-methoxybenzaldehyde)

Q36. Which laboratory precaution is important when working with cyanide catalysts?

  • Work in a fume hood and avoid acidification that can release HCN
  • Always warm mixtures to drive reaction to completion
  • Store cyanide solids next to strong acids for convenience
  • No special precautions are needed

Correct Answer: Work in a fume hood and avoid acidification that can release HCN

Q37. Which element of product analysis helps determine enantiomeric excess in asymmetric benzoin?

  • IR spectroscopy alone
  • Chiral HPLC or chiral GC analysis
  • Simple TLC with silica gel only
  • Melting point comparison without standards

Correct Answer: Chiral HPLC or chiral GC analysis

Q38. Which mechanistic step differentiates NHC-catalyzed benzoin from cyanide-catalyzed benzoin?

  • Formation of a metal-carbenoid intermediate
  • Formation of a nucleophilic Breslow-type intermediate via NHC addition to carbonyl
  • Use of radical chain propagation
  • Direct hydride transfer from solvent

Correct Answer: Formation of a nucleophilic Breslow-type intermediate via NHC addition to carbonyl

Q39. Which pharmacological relevance applies to benzoin derivatives?

  • They are major active sites in opioid receptors
  • They serve as intermediates in the synthesis of various bioactive molecules
  • They are used directly as antibiotics
  • They have no relevance and are only industrial plastics

Correct Answer: They serve as intermediates in the synthesis of various bioactive molecules

Q40. Which purification method is commonly used for benzoin after reaction workup?

  • Recrystallization from ethanol
  • Distillation under high vacuum only
  • Direct sublimation at room temperature
  • No purification is possible

Correct Answer: Recrystallization from ethanol

Q41. What color is benzoin typically as a solid?

  • Bright red
  • White to pale yellow crystalline solid
  • Deep blue
  • Colorless oil

Correct Answer: White to pale yellow crystalline solid

Q42. Which pH environment favors cyanide-catalyzed benzoin condensation?

  • Strongly acidic (pH < 2)
  • Neutral to mildly basic conditions
  • Extremely basic (pH > 13)
  • Completely anhydrous acidic conditions only

Correct Answer: Neutral to mildly basic conditions

Q43. Which byproduct would indicate over-oxidation during benzoin synthesis?

  • Benzaldehyde
  • Benzil (di-ketone)
  • Benzyl chloride
  • Toluene

Correct Answer: Benzil (di-ketone)

Q44. Which stoichiometry is correct for the classical benzoin condensation of benzaldehyde?

  • 1 benzaldehyde → 1 benzoin
  • 2 benzaldehyde → 1 benzoin
  • 3 benzaldehyde → 2 benzoin
  • 1 benzaldehyde → 2 benzoin

Correct Answer: 2 benzaldehyde → 1 benzoin

Q45. Which spectroscopic signal would confirm the presence of the benzylic hydroxyl proton in benzoin by 1H NMR?

  • A singlet at ~9–10 ppm
  • A broad singlet often between 4–6 ppm depending on solvent and hydrogen bonding
  • A triplet at ~0.9 ppm
  • No proton signal at all

Correct Answer: A broad singlet often between 4–6 ppm depending on solvent and hydrogen bonding

Q46. Which modification can improve yield in a sluggish benzoin condensation?

  • Decrease catalyst amount to near zero
  • Change to a more active catalyst such as an NHC or adjust solvent and temperature
  • Add a large excess of water
  • Remove all stirring

Correct Answer: Change to a more active catalyst such as an NHC or adjust solvent and temperature

Q47. Which heteroaromatic aldehyde is often used successfully in cross-benzoin reactions?

  • Pyridine-2-carboxaldehyde
  • Benzyl chloride
  • Cyclohexanone
  • Acetophenone

Correct Answer: Pyridine-2-carboxaldehyde

Q48. What is a practical advantage of NHC-catalyzed benzoin over cyanide-catalyzed methods for pharmaceutical labs?

  • NHCs are non-toxic and avoid cyanide hazards while providing similar reactivity
  • NHCs always give racemic products only
  • NHCs require stoichiometric metal co-catalysts
  • NHCs produce only polymers

Correct Answer: NHCs are non-toxic and avoid cyanide hazards while providing similar reactivity

Q49. Which reagent would you use to convert benzoin into an α-hydroxy acid derivative?

  • Reduction with LiAlH4
  • Oxidative cleavage under strongly oxidizing conditions to obtain carboxylic acids
  • Direct methylation with diazomethane
  • Hydrogenation over Pd/C without further oxidation

Correct Answer: Oxidative cleavage under strongly oxidizing conditions to obtain carboxylic acids

Q50. Which statement best summarizes the synthetic utility of benzoin condensation for B. Pharm students?

  • It is an obsolete reaction with no current applications in drug synthesis
  • It provides a mild method to form α‑hydroxy ketones, valuable building blocks in medicinal chemistry
  • It only produces toxic compounds unsuitable for pharmaceuticals
  • It is exclusively used for polymer synthesis

Correct Answer: It provides a mild method to form α‑hydroxy ketones, valuable building blocks in medicinal chemistry

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